1. Field of the Invention
This invention generally relates to devices for performing an analysis of optical radiation. More particularly, the present invention concerns an optical arrangement which may be employed to measure simultaneously and instantaneously the complex degree of coherence of optical radiation emanating from quasi-continuous (CW) sources and pulsed sources within its field of view. In addition, for quasi-monochromatic sources the device can be employed to measure mean wavelength as well.
The measurement of coherence of optical radiation is useful for many purposes. Coherence is an indicator of the bandwidth of a quasi-monochromatic source. The coherence of laser light is dependent on the mode properties of the laser. Thus, if the laser is operating in a single longitudinal and transverse mode, the coherence will be greater than if the laser is operating in two or more modes simultaneously. Thus, the invention in an appropriate embodiment will be able to discriminate instantaneously between multimode and single mode operation of a laser. This information is useful, for example, in holography. Because a certain minimum degree of coherence is necessary for holographic imaging. The ability to measure wavelength is especially useful. The invention can be used in conjunction with tunable laser systems to continuously monitor coherence and wavelength.
It will be readily understood by those skilled in the art, in view of the following detailed disclosure, that these methods and techniques for measuring the complex degree of coherence and wavelength are suited for employment in a great variety of applications. It is also to be understood that the invention is not to be limited in its manner of employment and many specific uses are contemplated although, for purposes of brevity, not enumerated separately in detail. It is specifically pointed out that the present invention comprehends employment of the methods and apparatus both alone and in combination with conventional apparatus such as prisms, lenses, gratings, laser devices, spectrographs, optical receivers, optical sources, and modulated optical communication links.
2. Description of the Prior Art
There are many well known methods for measuring the wavelength of an optical source. These include scanning monochrometers, spectrographs, and Fourier transform spectrometers. A Michelson interferometer can be used to measure the fringe visibility of a quasi-monochromatic source. However, a sequential measurement of the fringe pattern as a function of time is required. The fringe pattern must then be analyzed to calculate the complex degree of coherence as a function of path difference. Whereas the invention to be disclosed in the following sections can instantaneously and continuously measure the variables necessary for computing either by analogue or digital methods the complex degree of coherence. It will be obvious to one skilled in the art that this technique is a significant improvement over prior art. Techniques have been developed for measuring the coherence length of a single pulse of optical radiation (see for example, Dandliker et al. U.S. Pat. No. 3,776,636 and R. A. Patten, U.S. Pat. No. 3,764,217). These techniques require the use of photographic film or other recording medium for measuring coherence length. Whereas the present invention disclosed in the following discussion can measure the complex degree of coherence instantaneously and without the need for the development of photographic film. This advantage over prior art is especially important and can be used to advantage where it is desired to measure the width of an absorption or emission line of optical radiation as a function of experimental parameters such as pressure, temperature, or electric field.
Thus a need exsists for a technique and apparatus for measuring the complex degree of coherence and wavelength on a continuous and instantaneous basis.